The present invention relates to a relay authorizing or interrupting the flow of a current in a load circuit. It also relates to the application of this relay to a load circuit in which a random direction or alternating current flows.
At present two types of relay are known, namely electromechanical relays and static electronic relays.
Electrochemical relays comprise a control electromagnet, whose moving armature controls the opening or closing of contacts. These relays are essentially defined by their interrupting or breaking power, i.e. the maximum load current I and the maximum load voltage V, their control quantity, i.e. the a.c. or d.c. current I or voltage V flowing in the control electromagnet and their insulation voltage between the load circuit and the electromagnet control circuit.
The essential advantage of these relays is that they can be inserted at any random point in an existing electric circuit, provided that their limiting characteristics V. I, v, i as well as the insulation voltage are not exceeded. They also make it possible to separate the control current and the load current, whilst being simple and inexpensive.
However, they suffer from disadvantages which are difficult to overcome. Thus, they are slow (due to their mechanical inertia), their switching time generally being a few dozen milliseconds in the case of power relays. They produce breaking sparks, which lead to rapid wear to the contacts, which can also corrode. Thus, designers only guarantee the relays produced by them for a limited number of switching operations, under clearly defined operating conditions. They are noisy, generate vibrations, sensitive to shocks and consume a certain electric power, even though the load or use current is zero.
Most known static electronic relays are not available in the form of independent components, which can be incorporated into a random load circuit. Generally, these static relays are special circuits included in an electronic assembly having the function of a relay and used at the same time as the other components of said assembly. No relays exist which use static components having certain of the advantageous characteristics of electromechanical relays, so that they can be substituted for an electromechanical relay between two random points of an existing electronic assembly, without leading to modifications to the latter.
Thus, the construction of an electronic static relay requires the supply of its components, which raises significant difficulties if the voltage to be switched is of a floating nature with respect to a reference earth. The known static relays also cause insulation and leakage current problems between the load circuit and the control circuit. Moreover, they function very inadequately in the vicinity of a zero load current. They cannot be switched at a very high frequency.
Recently an independent electronic static relay with photoelectric coupling has become available, but said relay has a very specific use range and does not operate at zero currents or voltages.
The known static relays having these disadvantages can comprise either thyristors or transistors. Thyristor static relays use an insulated supply circuit and do not operate correctly when the load current passing through the transistor drops below one tenth of the nominal value of said current.
The transistor static relay also uses an insulated supply, if it is wished to introduce said relay between two random points of a load circuit.